A printing apparatus which prints information such as a desired character or image on a sheet-like printing medium such as a paper sheet or film is widely used as an information output apparatus in a wordprocessor, personal computer, facsimile apparatus, and the like.
Various methods are known as printing methods for the printing apparatus. Especially an inkjet method has recently received a great deal of attention because this method can realize noncontact printing on a printing medium such as a paper sheet, easily prints in color, and is quiet. Because of low cost and easy downsizing, a popular inkjet arrangement is a serial printing system in which a for discharging ink in accordance with desired printing information prints while reciprocally scanning in a direction perpendicular to the convey direction of a printing medium such as a paper sheet.
FIG. 11 is a block diagram showing the representative circuit configuration of a conventional inkjet .
In FIG. 11, reference numeral 101 denotes an electrothermal transducer (heater) for generating thermal energy; 102, a power transistor for supplying a desired current to the heater; 103, a shift register which temporarily stores printing data DATA for determining whether to discharge ink from the nozzle of the in accordance with image information to be printed; 104, a transfer clock input terminal which is attached to the shift register and inputs a transfer clock signal CLK; 105, a printing data input terminal for serially inputting the printing data DATA to the shift register; 106, a latch circuit for latching printing data stored in the shift register; 107, a latch signal input terminal for inputting a latch signal LT for controlling the latch timing of the latch circuit 106; 108, a power line for applying a predetermined voltage (VH) to the heater and supplying a current; and 109, a GND line serving as the reference of power or an applied voltage.
FIG. 12 is a timing chart showing various signals for driving the shown in FIG. 11. Reference numeral 201 denotes a transfer clock CLK; 202, printing data DATA; 203, a latch signal (LT); and 204, a heat enable signal HE.
The transfer clock (CLK) pulse 201 is input to the transfer clock input terminal 104. The printing data (DATA) 202 representing ON/OFF of each heater is serially input from the data input terminal 105 so that printing data is transferred to the shift register 103 in synchronism with the two edges of the transfer clock 201. After data is transferred to the shift register 103, the latch 106 latches printing data corresponding to each heater at a timing at which the latch signal (LT) 203 is input to the latch input terminal 107.
At an appropriate timing, the heat enable signal (HE) 204 is supplied. A current flows through the power transistor 102 and heater 101 in accordance with a time during which the heat enable signal is ON (in this example, low level), and ink is discharged in accordance with printing data. If necessary, a time during which the heater is driven may be changed depending on the temperature and the number of simultaneously driven heaters (number of simultaneous ON bits).
In FIG. 12, a pre-pulse 205 is supplied immediately before the heater is driven by the heat enable signal 204. This is based on a technique disclosed in U.S. Pat. No. 6,139,125 (corresponding to Japanese Patent Laid-Open No. 5-31906). This technique intends to keep a at a high temperature and stabilize the ink discharge amount by supplying the pre-pulse 205. The pre-pulse application time is short enough not to discharge ink.
U.S. Pat. No. 6,520,613 (corresponding to Japanese Patent Laid-Open No. 9-327914) discloses an arrangement which decodes signals input from a plurality of signal lines to generate a block selection signal in order to decrease the number of input terminals and improve the reliability.
Recently, inkjet printers are achieving multicolor printing, higher speeds, and higher image qualities, and the printing data amount tends to increase. The number of signals necessary to drive the  and the number of input terminals also tend to increase. An increase in the number of input terminals leads to a decrease in connection reliability and an increase in chip area, raising the chip cost.
Since an increase in cost raises the cost of the whole apparatus and the running cost, the number of input terminals is desirably decreased.